Radio receivers employ different channel equalizers to remove intersymbol interference (ISI), which is caused by linear and non-linear distortions to which a signal is subjected in a radio channel. Intersymbol interference occurs in band-limited channels when the pulse shape used spreads to adjacent pulse intervals. The problem is particularly serious at high transmission rates in data transfer applications. There are many different types of equalizers, such as a DFE (Decision Feedback Equalizer), an ML (Maximum Likelihood) equalizer and an MLSE (Maximum Likelihood Sequence Estimation Equalizer), the two latter ones being based on the Viterbi algorithm.
It is widely known that the information received from equalizers based on the Viterbi algorithm for soft decision making in decoding must be weighted taking noise or interference power into account in order to enable the performance to be optimized. The problem is then how to estimate the noise power in a reliable manner.
Publication U.S. Pat. No. 5,199,047 discloses a method which enables reception quality to be estimated in TDMA (Time Division Multiple Access) systems. In the method, channel equalizers are adjusted by comparing a training sequence stored in advance in the memory with a received training sequence. A training sequence is transmitted in connection with each data transmission. The publication discloses a widely known receiver structure wherein impulse response H(O) of a channel is determined by calculating the cross-correlation of received training sequence X′ with sequence X stored in the memory. This impulse response controls a Viterbi equalizer. The publication discloses a method which enables the reception quality to be estimated by calculating estimate S for a received signal                               S          =                                                    ∑                0                i                            ⁢                                                           ⁢                              s                i                                      =                                          ∑                0                I                            ⁢                                                           ⁢                                                                                                            y                      i                                        -                                          x                      i                      ′                                                                                        2                                                    ,                            (        1        )            
wherein
yi is the calculated estimate for a signal (including a training sequence) transmitted without interference, and
xi ′ is the received sample.
The lower estimate S is, the higher the correlation of the estimated training sequence with the received signal sample. Hence, the lower estimate S is, the higher the likelihood that the transmitted data bits can be detected by the channel equalizer used.
The publication also discloses a relative estimate, i.e. quality coefficient Q, which takes the power of the received signal into account                               Q          =                                                    ∑                                                                                                X                      i                      ′                                                                            2                                            S                        =                                          ∑                                                                                                x                      i                      ′                                                                            2                                                            ∑                                                                   ⁢                                                                                                                        y                        i                                            -                                              x                        i                        ′                                                                                                  2                                                                    ,                            (        2        )            
wherein quadratic values of training sequence Xi ′ or individual sample values xi ′ are summed in order to determine received signal energy.
A receiver usually, e.g. in a GSM (Global System for Mobile Communications) system modification called EDGE (Enhanced Data Services for GSM Evolution), comprises prefilters before the channel equalizer. Publication U.S. Pat. No. 5,199,047 does not disclose how this fact can be utilized in optimizing the channel equalizer.